In a double slit experiment, interference is obtained from electron waves produced in an electron gun supplied with voltage V. If $\lambda$ is wavelength of the beam, D is the distance of screen, d is the spacing between coherent sources, ft is Planck's constant, e is charge on electron and m is mass of electron, then fringe width is given as

• A

  $\frac{\mathrm{hD}}{\sqrt{2\mathrm{meVd}}}$

• B

  $\frac{2\mathrm{hD}}{\sqrt{\mathrm{meVD}}}$

• C

  $\frac{\mathrm{hd}}{\sqrt{2\mathrm{meVD}}}$

• D

  $\frac{2\mathrm{hd}}{\sqrt{\mathrm{meVD}}}$